This invention relates to keyboard electronic musical instruments and, more particularly, to a digital tone generating system of the waveshape memory type.
In known keyboard electronic musical instruments of the waveshape memory type, memories having the required waveshapes stored therein are read out by a read-out address signal corresponding to a depressed key of the keyboard, the resulting derived waveshapes are multiplied with an envelope wave which controls the amplitude of the tone to be produced, and the resulting signal converted to analog form by a digital-to-analog converter. Examples of note generators of the waveshape memory type are described in U.S. Pat. Nos. 4,202,234 and 4,224,856, two of many patents directed to various aspects of digital tone generation.
In prior note generators of this type the address signal typically is eight bits in length, or capable of addressing one of 256 samples, and usually some of the bits, although having significance, are treated as if they were insignificant and discarded. As a consequence, the waveshape read out of memory contains quantization errors which can result in audible noise in the generated tone signal. The source and nature of the error will be seen from examination of FIG. 1 which illustrates the digitizing of a sinewave by classic digital-to-analog conversion techniques wherein the value of each sample is considered valid from each sampling time point until the next, whereupon switching to the next value occurs and held until the next successive sampling time point. It is seen that this process produces a stepped error function; although the use in FIG. 1 of only sixteen sample points emphasizes the error function and is, of course, much less pronounced when 256 samples are used, the error nonetheless exists and adversely affects the musical quality of the generated tone signals. The signal quality could be improved by making the steps in the error function smaller, but this would be at the expense of increased complexity and cost of longer wave tables. Accordingly, a primary object of the present invention is to provide a musical note generator of the waveshape memory type in which the error function is reduced as compared to prior systems, yet requires no more memory capacity.